1. Field of the Invention
The present invention relates to an induction heating fixing device that is loaded in an image forming apparatus such as a copy machine, printer or facsimile and that fixes a toner image to a paper by using a heating member heated by induction heating.
2. Description of the Background
As a fixing device used in an image forming apparatus such as an electrophotographic copy machine or printer, there is a device that inserts a sheet of paper between a pair of rollers including a heat roller and a pressurizing roller or into a nipping part formed between similar belts, and then fixes a toner image by heating and pressurizing. As such a heating-type fixing device, there is an induction heating fixing device that heats a metal conductive layer on the surface of a heat roller or a heating belt by an induction heating method in order to realize a higher process speed. In the induction heating method, predetermined power is supplied to an induction heating coil to generate a magnetic field there, and the metal conductive layer is instantaneously heated by an eddy-current generated in the metal conductive layer by the magnetic field. Thus, the heat roller or heating belt is heated.
As a fixing device based on the induction heating method as described above, there traditionally is a device in which the induction heating coil is divided into plural parts to realize even temperature distribution in the longitudinal direction of the heat roller. For example, JP-A-09-106207 or JP-A-2001-185338 discloses an induction heating fixing device in which plural induction heating coils, divided as plural parts in the longitudinal direction of the heat roller, are selectively driven to realize even temperature of the heat roller across the total length in the longitudinal direction.
However, in all of these traditional induction heating fixing devices, there is a gap at a joint part between the neighboring induction heating coils. Therefore, the magnetic field changes at the joint part and may cause uneven temperature in the heat roller. Moreover, in the traditional induction heating device, since the divided plural induction heating coils are selectively driven, for example, when the heat roller as a whole is to be supplied with 1000 W, each area of the divided induction heating coils requires 1000 W. For example, to selectively drive two induction heating coils and heat the heat roller across its total length, each of the two induction heating coils must be supplied with 1000 W. Therefore, the electrical quantity per unit area of the heat roller is large. This can be a factor that increases temperature variance in the heat roller corresponding to each induction heating coil.
In the case where the plural induction heating coils are simultaneously driven in order to reduce temperature variance in the heat roller, since the plural induction heating coils have driving frequencies from each other, interference noise occurs at the time of driving and it additionally causes the risk of increased noise. Moreover, the traditional induction heating fixing device is not configured in consideration of improvement in the uneven temperature between a paper passing part and a non-paper passing part due to the size of a sheet of paper. Therefore, there is a risk that the temperature of the non-paper passing part may be raised by continuous paper passing and adversely affect peripheral devices.
Thus, in the fixing device in which the metal conductive layer is heated by the induction heating coil, the occurrence of uneven temperature in the longitudinal direction of the heat roller is to be prevented. Particularly, development of an induction heating fixing device is desired in which the temperature rise in the non-paper passing part due to the size of a sheet of paper is prevented to realize a stable fixing property.